In time-division multiple access (TDMA) communication systems, each frame is an interval of T seconds, and each frame is divided into n discrete time slots. Thus, subscriber units can communicate with each other on a basis of non-overlapping transmission bursts. Since there is no overlap, a same pair of carrier frequencies, one for uplink and one for downlink, may be assigned to all subscriber units using a same base site.
The TDMA technique is characterized by duration of the time frame and the time slot within the frame. Each time slot typically consists of a guard time, a preamble, and the information to be transmitted. Typically, the preamble contains system information such as synchronization, control and routing information. The guard time and the preamble generally depend on the organization of the system. The information to be transmitted occupies a predetermined number of bits.
Some time division multiple access, i.e., TDMA, systems, allow contention for the use of one slot to occur across multiple slots. In this type of system, subscriber unit terminals may effectively be designed to be half-duplex. For example, subscriber units may attempt to increase efficiency for a wireless system that is primarily intended for circuit-switched traffic by using excess system capacity for packet-switched data services via similar subscriber unit equipment. While the equipment may be full-duplex, said equipment may be incapable of transmitting and receiving data on two different time slots since this is not required for full-duplex circuit mode communication in a TDMA system. In this system, the incidence where a particular subscriber unit is transmitting a packet of data on a first time slot, and a base station is transmitting a packet to the same subscriber unit on a second time slot, is called a collision. When collision occurs, at least one of the two transmitted packets will be lost. Thus, retransmission of the lost packet is required, and system throughput is reduced.
Depending upon the type of packet services being used, the number of time slots, and the number of subscriber units, the collisions tend to significantly degrade system performance. Use of a reservation contention algorithm may add a significant amount of delay to the system. Thus, there is a need for a collision avoidance method and system that prevents collision of transmitted packets, thus alleviating the necessity for retransmission of lost packets and increasing system throughput.